1. Field of the Invention
The present invention relates to a handwritten character recognition method, and more particularly to a signature verification method and to an apparatus for verifying the authenticity of a signature through combination of writing pressure information and coordinate information relating to handwritten characters.
2. Description of the Prior Art
A handwritten character recognition method by which written characters are recognized has been utilized as an input method for word processors or a signature verification method for specifying a writer. Under a handwritten character recognition method which has already been in actual use as an input method, characters are input in the block style under specified constraints on the style of typeface, and the thus-input characters are converted into coordinate information. The thus-converted coordinate information is verified by comparison with coordinate information relating to character data which have been stored beforehand. As a result of verification, the characters are recognized as matched. If characters are carefully written in the block style at comparatively slow speed, the characters can be sufficiently recognized through use of only coordinate information because under such conditions each of the strokes of the characters becomes clear by virtue of visual feedback to the writer and hence the shape of the characters becomes stable.
In contrast, in a case where the character recognition method is applied to an input method which does not pose any restriction on the style of typeface at the time of input of characters or to a signature verification method, there must be recognized not only characters written in the block type but also cursively written characters. When characters are cursively written, writing movement becomes faster and does not involve any substantial visual feedback to the writer. In this case, the characters become less identifiable, and separation of a resultantly acquired pattern into strokes becomes difficult. This is because of greater expansion or contraction of the pattern in the direction of the time axis or in the direction of stroke, or the difference between the input pattern and a pre-registered pattern. For this reason, an extremely low matching rate is obtained through use of only the coordinate information, rendering the characters unidentifiable.
Another method is to enable recognition of characters without involving the separation of characters into strokes by application of coordinate information and writing pressure. This method employs a pattern matching technique called dynamic programming (DP) matching, which takes into consideration variations in the coordinate information stemming from variations in writing action.
In the dynamic programming matching technique, variations in the writing movement are corrected with regard to the time axis or the arc length axis through use of a warping function which minimizes a cumulative difference between patterns to be checked. Patterns are matched with each other on the basis of the coordinates and writing pressure that have been corrected so as to compensate variations in the writing movement, thereby enabling recognition of cursively handwritten characters.
Verification based on the addition of writing pressure information to coordinate information or normalization of input patterns by DP matching contributes to an improvement in the recognition rate of handwritten characters. However, in the case of application of the dynamic programming matching technique to recognition of cursively written characters or signature verification, a false signature may be erroneously recognized as a genuine signature. Therefore, in its present form, the dynamic programming matching technique cannot be put into practical use.
Japanese Patent No. 1,822,532 [Japanese Patent Publication (kokoku) No. 5-31798] entitled "A Method of Recognizing Handwritten Characters Online" describes a practical technique that is based on dynamic programming matching. Under this method, when the degree of difference between a registered pattern and an input pattern of handwritten characters is calculated by use of the dynamic programming matching technique, coordinate information and writing pressure information are simultaneously processed by the assignment of optimum weighting coefficients to the coordinate information and writing pressure information. As a result, the difference is reduced, which in turn contributes to an improvement in the verification rate of authenticity and a reduction in processing time.
As mentioned previously, even in the case of unclear characters which cannot be separated into strokes, processing of the coordinate information and writing pressure information relating to handwritten characters enables recognition of the characters. Further, even in the case of cursively handwritten characters, the characters can be recognized in practice, as a result of a further improvement in the dynamic programming matching technique that corrects the cumulative difference so as to compensate variations in writing movement.
However, there is a case where even the foregoing method fails to compensate variations in writing movement, to thereby cause recognition errors. FIG. 1 is a diagrammatic representation showing examples of character strings written by the same writer. FIG. 1(1) shows a character string which is registered as the reference for verification, and FIG. 1(2) shows an example input character string. As can be seen from the drawings, the pitch between characters is likely to change depending on a change in the environment where the writer writes characters. If the change in pitch between the characters is significant, the change cannot be compensated by correction of the position or size of the characters and is accumulated as an error of coordinates, thereby reducing the verification rate.